Blockage Ratio Research Articles

Impact of mixture mass flux on hydrodynamic blockage ratio and Mach number of rotating detonation combustor

To analyze non-ideal phenomena, such as burned gas backflow and non-detonation combustion, which affect the rotating detonation wave Mach number, simultaneous self-luminous visualization, time-averaged static pressure, fluctuating pressure, and thrust measurements with gaseous ethylene and oxygen were performed. Consequently, by doubling the number density of the fuel injectors, the hydrodynamic blockage ratio at the oxidizer inlet increased approximately 1.7-fold under the same oxidizer inlet area conditions. This may be attributed to the increase in the detonation propagation Mach number owing to the enhanced mixing of fuel and oxidizer. The relationship between the parasitic combustion fraction in front of the rotating detonation wave and the Mach number was also investigated by using a distributed heat release model. Consequently, it was suggested that experimental Mach number decreased from approximately 4.1 to 2.8 with increase in a mixture mass flux, and the theoretical detonation wave propagation Mach number was 7.3.

Leakage Flow Impact on Shrouded Stator Cavity Flow Topology and Associated High-Speed Axial Compressor Stage Performance

AbstractThis paper presents the investigation of realistic cavity leakage flow for the von Karman Institute for Fluid Dynamics H25 axial compressor stage, equipped with a shroud cavity of average Reynolds number Rer = 2.2 * 106 and average aspect ratio G = 0.06. On top of overall performance measurements, time averaged data from unprecedented experimental dataset are used to characterize the stator hub shroud cavity flow field and its interaction with the power stream. The sensitivity of the stage performance and stability as well as the cavity flow topology is evaluated against injection conditions, compressor throttling, and compressor speed. Parametric steady RANS simulations of the stage under injection are used to support the findings and state on the relevance of such approach in the preliminary design phase of axial compressor components. As an example, a stage efficiency reduction of 0.97% for a leakage fraction of 0.56% at design point is retrieved. This performance drop is attributed to the effect of cavity flow on: the blockage ratio, the boundary layer skewness, and total temperature increase at stator row inlet. It is also presented that there is a mutual interaction between the cavity geometry and the cavity flow field organization. The injection homogenizes the cavity flow field and the associated pressure gradient. The various operating conditions presented also demonstrate that the cavity flow studied is, on average (i.e., time), sensitive to changes in compressor rotation speed and changes in stage loading.

Impact of Flow in Pipe and Box Coverages on Open Channel’s Performance

This experimental study aims to characterize the behavior of pipe and box coverages concerning their size, form, and upstream blockage ratio. In an artificial trapezoidal cross-section, five experimental cases were carried out: Case 1 involved an artificial canal without coverage or blockage; Cases 2 and 3 concerned pipe coverages with circular cross-sections (Pipe 1 and 2); Cases 4 and 5 involved box coverages with square cross-sections (Box 1 and 2). While the area of pipe 1 and box 1 are equal, the area of pipe 2 and box 2 is the same and greater than the area of pipe 1 and box 1. Three blockage ratios and three water flow rates were used in the experimental study. Each case included an investigation of the hydraulic performance of the open channel and the scouring pattern downstream of the coverage. In comparison to the case when there was no coverage present at the same condition, the presence of coverage in the open channel and the significant increase in flow rate, blocking ratio, and decreasing inlet area of coverage increased the heading up, head losses, scour depth, and scour length. In the same area and condition, the pipe coverage achieves greater scour depth and length than the box coverage. The box coverage is better for the open channels' performance than the pipe coverage. The research recommended using the box coverage rather than the pipe coverage and checking the maintenance processes to avoid the negative effect on the open channels' performance.

Self-organizing single-line particle trains with differently shaped particles in a channel flow

The inertial migration of differently shaped rectangular particles and elliptical particles in a channel flow and the self-organization of single-line particle trains are studied using the lattice Boltzmann method. The effects of particle shape, particle aspect ratio (α), Reynolds number (Re), blockage ratio (k), and particle concentration (Φ) on self-organizing single-line particle trains are explored. The results show that a single-line particle train is dynamically formed, with circular particle trains having a more pronounced dynamic process than rectangular and elliptical particle train. The inclination of height (IH) for the particles in the train is the main reason for the dynamic formation of a single-line particle train. Due to the changes of orientation angle under different flow conditions, the rectangular particle trains always have a larger IH and smaller interparticle spacing than the elliptical particle trains when the train is just formed. The effect of α on the spacing of elliptical particle trains is more sensitive than other shapes. Rectangular particles and elliptical particles with large Φ and Re and small k are prone to self-organize the single-line particle trains with stable spacing for a long travel distance. With increasing Φ, Re, and k, IH increases and the interparticle spacing decreases.

Adaptive simulations of flame acceleration and detonation transition in subsonic and supersonic mixtures

Two-dimensional simulations were carried out to investigate the flame acceleration and deflagration-to-detonation transition (DDT) in a combustion chamber filled with a subsonic or supersonic mixture by employing Navier-Stokes equations together with a detailed chemistry reaction mechanism of 11 species and 27 steps under adaptive mesh refinement. The effects of the initial mixture Mach number and mesh resolution on the flame acceleration and DDT were studied in detail, and the entire processes of the flame acceleration, DDT and detonation propagation were revealed. Two DDT mechanisms are obtained in a chamber having the same low blockage ratio but with different initial velocities of the mixture. Regime I: multiple shock wave collisions, shock focusing and shock reflection result in a rapid energy deposition in a small region; a direct detonation subsequently occurs in the boundary wall for the subsonic mixture. Regime II: the classic hot-spot mechanism due to the reactive gradient mechanism is responsible for the detonation transition in the supersonic mixture when the intense leading shock wave impacts and reflects on the solid surface. By increasing the initial mixture Mach number, the run-up time and distance to DDT are dramatically reduced. The flame front structure and propagation in the supersonic flow demonstrate that the detonation cell size rapidly increases when propagating into the smooth region due to detonation attenuation and resulting pressure decrease. Additionally, a much higher combustion temperature occurs in the upper and lower walls because of the Mach stem. In comparison, the results show that the detonation overdrive degree and pressure gain ratio in the subsonic mixture are higher than in the supersonic mixture. Moreover, flame propagation upstream also suggests that increased pressure and temperature occur in the inlet isolation section, even forming a localized explosion point.

Asymmetric wakes in flows past circular cylinders confined in channels

In this paper, we investigate the flow past a circular cylinder confined in a channel at a blockage ratio of $\beta =0.7$ (the ratio of the cylinder diameter and the channel height) for Reynolds numbers between ${\textit {Re}}=300$ and $3900$ using direct numerical simulation (DNS). We show for varying Reynolds numbers a wide range of wake dynamics occur as the spanwise domain length is changed. At a lower Reynolds number of ${\textit {Re}}=300$ , a reverse von Kármán wake alongside either a top- or bottom-biased asymmetry was observed at different spanwise locations. The asymmetry was structurally similar the two-dimensional asymmetry studied by prior investigators, and was found to be a result of the confinement effect. Further, wake-jumping between the two intermittent states was present. For larger Reynolds numbers, ${\textit {Re}}=1000$ and $3900$ , these asymmetric structures were found to become dominant. We also examine the dependence of the asymmetries on the spanwise domain. For small spanwise domains the asymmetries were uniformly orientated across the span. In contrast, for sufficiently large spanwise domains, the asymmetry flips its orientation at different spanwise locations. Comparisons of flow statistics demonstrate good agreement between the different spanwise domains, which suggests the same mechanism maintains the asymmetry in both cases. Further analysis at ${\textit {Re}}=1000$ found the number of times the wake flips is dependent on the initial conditions, with a wake that flips zero (purely asymmetric), two and four times being observed. These structures were also determined to remain stable over time scales of $1000D/U$ .

Effect of the number of ribbed walls on the thermal performance in rectangular channels with 45-deg parallel or criss-cross ribs

Regionally averaged heat transfer distributions and friction characteristics are experimentally investigated for fully developed, turbulent flow in a rectangular channel with a width-to-height aspect ratio (AR=3). 45-deg angled rib turbulators are placed on one, two, and four sides of the channel in parallel and criss-cross configurations with a pitch-to-height ratio(P/e=10) and a rib blockage ratio(eDh=4.1%). For a roughness Reynolds number range(e+=25−500), results show that increasing the number of ribbed walls improves the overall channel heat transfer for the 45-deg parallel ribs. Results reveal that four-sided ribbed channels provided the greatest thermal performance followed by the two-sided and one-sided ribbed channels, respectively, for both angled rib designs. On the basis of the law of wall similarity, friction and heat transfer roughness functions are derived using the four-sided data reported in this work. Furthmore, friction factors and heat transfer coefficients of similar angled rib designs were obtained from open literature and similarly correlated to derive more robust friction and heat transfer correlations over the roughness Reynolds number range. Thermal designers can use these proposed correlations to predict friction and heat transfer in rectangular cooling channels (AR=1-5) with various numbers of ribbed walls. The measured friction factor and heat transfer coefficient of this study can be predicted using these semi-empirical correlations with maximum errors of 9% and 7%, respectively.

Numerical simulation of interaction of cellular detonation wave with systems of inert porous filters

Numerical modeling of the interaction of cellular detonation in a hydrogen-air mixture with several systems of porous filters covering part of the channel was carried out. The main regimes of detonation propagation and the critical conditions for detonation failure in the filter systems were obtained for each system. A map of detonation regimes was constructed, from which it follows that with an increase in the concentration of particles in the filters, it is possible to increase the distance between the filters or reduce the number of filters in the system necessary to successfully suppress detonation. A comparison of various filter systems in terms of blockage ratio and the total surface area of particles was made, from which it was concluded that the system of two filters was the most efficient to suppress detonation.

An overpressure-time history model of methane-air explosion in tunnel-shape space

This study investigated methane-air explosion in tunnel-shape space and developed an overpressure-time history model based on numerical results. The findings revealed that for the progressively vented gas explosion with movable steel obstacles in a 20 m long tunnel, the inner peak overpressure increased as the activation pressure of the tunnel top cover got higher but remained below 6 bar. However, as the activation pressure increased to 8 bar or higher, the peak inner overpressure remained unchanged. As the segment cover panel became wider, the peak pressure was almost unchanged, but the pressure duration and impulse declined significantly. The peak pressure and impulse increased as the tunnel length vary from 10 to 30 m. With fixed tunnel length, higher blast pressure but lower impulse was observed as the inner obstacles were closer or the activation pressure of obstacles was higher. It is also found that a local enlarged space in the tunnel enhanced the peak pressure significantly. An overpressure time history model for the tunnel with fixed top cover and enlarged end zone was established. The model considered activation pressure of vent cover, area and length of vent opening, methane concentration, number and blockage ratio of fixed obstacles was developed to calculate the overpressure and corresponding time at characteristic points of the pressure-history curve. The cubic Hermite interpolation algorithm and a specially tuned formula consisting of the power and exponential function were used to interpolate pressure values between characteristic points. The proposed model can predict both the peak pressure and the overpressure time history with acceptable accuracy.

Multi-fidelity modelling of shark skin denticle flows: insights into drag generation mechanisms.

We investigate the flow over smooth (non-ribletted) shark skin denticles in an open-channel flow using direct numerical simulation (DNS) and two Reynolds averaged Navier-Stokes (RANS) closures. Large peaks in pressure and viscous drag are observed at the denticle crown edges, where they are exposed to high-speed fluid which penetrates between individual denticles, increasing shear and turbulence. Strong lift forces lead to a positive spanwise torque acting on individual denticles, potentially encouraging bristling if the denticles were not fixed. However, DNS predicts that denticles ultimately increase drag by 58% compared to a flat plate. Good predictions of drag distributions are obtained by RANS models, although an underestimation of turbulent kinetic energy production leads to an underprediction of drag. Nevertheless, RANS methods correctly predict trends in the drag data and the regions contributing most to viscous and pressure drag. Subsequently, RANS models are used to investigate the dependence of drag on the flow blockage ratio (boundary layer to roughness height ratio), finding that the drag increase due to denticles is halved when the blockage ratio δ/h is increased from 14 to 45. Our results provide an integrated understanding of the drag over non-ribletted denticles, enabling existing diverse drag data to be explained.

Numerical investigation of flow past a circular cylinder modified with a single groove at low Reynolds number

Flow past a circular cylinder modified by grooves is one of the passive control techniques to control the drag and lift forces. The presence of grooves tends to reduce the hydrodynamic drag and lift. The aim of this work was to numerically investigate the influence of a single groove modification on a circular cylinder and how it was impacting the hydrodynamic coefficients based on its position and shape at low Reynolds numbers. Three types of grooves were considered, namely, dimple, square, and triangular, with identical width and depth which is 0.1 times the diameter. The Reynolds numbers considered were 100, 150, and 200. The computational domain was set to a blockage ratio of 0.01 after carrying out a detailed domain independence test. It was observed that the drag and lift reduction was maximum at groove location 90° with the decrements being more for the square groove shape. The viscous drag showed a decrease in its values for all the groove positions located on the flow-facing side of the cylinder; however, the pressure drag showed an increment in its values for these groove locations, and these increments were relative of larger magnitudes, thus influencing a net increase in the total mean drag when compared to the smooth case; this was evident at groove locations 120° and 150°. The vorticity patterns for the grooves located on the front side showed large irregularities in magnitude between the anticlockwise and the clockwise vortices with the groove side vortices being of larger magnitudes, and for the wake side, the irregularities were negligible. The coefficient of pressure and separation angles were also investigated. The CP vs θ plots showed discontinuity at groove locations with abrupt variations at the front face of the cylinder; for the wake side grooves, these discontinuities were low and almost coincided with the CP vs θ plots of the smooth cylinder. The grooves located at 90° showed a delay in separation and at 60° the boundary layer tends to detach earlier.

Temporal evolution of flow field structure for vehicles accelerating in evacuated tube transportation system

As a supersonic transportation system, the flow around a vehicle in an Evacuated Tube Transportation (ETT) system will evolve through series of flow structures during acceleration. The occurrence of choked flow and shock wave will especially lead to the drastic change in flow field structures. In this study, based on the one-dimensional inviscid flow assumption, a theoretical model is established to quantitatively describe the formation time, formation location of choked flow, and Frontal Normal Shock Wave (FNSW), as well as the distance of the disturbed flow field region ahead of the vehicle in the ETT system. It is found out that the formation time of FNSW as well as the initial distance between the vehicle head and FNSW is linearly proportional to the blockage ratio while linearly inverse proportional to the acceleration rate of the vehicle. An experimentally verified numerical model is also established with an overset mesh technique to investigate the flow field evolution for vehicles accelerating in the ETT system. The results from numerical analysis agree well with the theoretical model. Meanwhile, five typical flow field structures are summarized for a vehicle accelerating from a stationary state to supersonic state in the ETT system. The applicability of the wind tunnel method and overset mesh technique in numerical simulation of the ETT system is systematically discussed. The influence of occurrence and dissipation of choked flow and shock wave on the vehicle's aerodynamic drag profile are then analyzed quantitatively.

Rigid spheroid migration in square channel flow of power-law fluids

The characteristics of rigid spheroid migration in square channel flow of power-law fluids are numerically investigated by the three-dimensional lattice Boltzmann method. The effects of aspect ratio (α), blockage ratio of the particle (k), power-law index (n) and Reynolds number (Re) of the fluid on the inertial migration of rigid prolate and oblate spheroids are explored, respectively. The results show that the shear-thinning fluid with large fluid inertia is beneficial for fast focusing rigid particles to the equilibrium positions in square channel. There are two stages of particle migration and four stable channel face equilibrium positions for migration of spheroid; the channel corner equilibrium positions only exist for the oblate spheroid under a low inertia effect. The prolate spheroid exhibits tumbling and log-rolling (LR) rotational modes, and the LR mode is conditionally stable. For the oblate spheroid, only the LR mode exists when Re is large while two new rotational modes are captured when Re decreases. The equilibrium position of the prolate and oblate spheroids increase when α < 1.0 then decrease at larger α values, escaping the channel centreline with decreasing n and k and increasing Re. The changes of inertial focusing length, angular velocity, and rotational period of the spheroid are systematically analysed to study mechanism of spheroid migration. The current results enrich our understanding of rigid particle accumulation behaviour in channel flow of non-Newtonian fluids and may also shed light on how to efficiently focus and control particles in microfluidic devices.

Experimental investigation on the enhancement of plenum window noise reduction using solid scatterers.

The sound transmission across plenum windows installed with rigid non-resonant cylindrical scatterer arrays was investigated in detail using scale-down model measurements carried out inside a fully anechoic chamber. The arrays have manifested to some extent the acoustical behaviors of virtual sonic crystals. The maximum cross section blockage ratio was 0.6. The effects of plenum window gap, array configuration, and scatterer diameter on the sound transmission characteristics were also examined. Results indicate that the window cavity longitudinal modes and the gap modes control the sound transmission characteristics at low frequencies. The upper bound of this frequency range increases with decreasing gap width. Within this frequency range, the scatterers have negligible effect on the sound transmission. At higher frequencies, the array configurations with scatterer(s) attached to the window walls result in stronger sound reduction. There are relatively higher sound transmission loss improvements around the frequencies where a full bandgap is observed. There are wide bandgaps in various lattice directions, and the present results suggest that they play a role in the broadband improvement of sound reduction.

Study on the Effect of Blockage Ratio on Maximum Smoke Temperature Rise in the Underground Interconnected Tunnel

The model-scale tunnel is used in this investigation to analyze the maximum smoke temperature rise of the interconnected tunnel for various longitudinal ventilation velocities, blockage ratios, and heat release rates where the fire is at the confluence of the underground interconnected tunnel. The results showed that the longitudinal ventilation velocities of both the ramp upstream of the fire source and the adjacent ramp influenced the maximum temperature rise under the underground interconnected tunnel, and the ventilation of both ramps jointly affected the maximum temperature rise. The change in the maximum temperature rise depends on who is more affected by the longitudinal ventilation velocity or the vehicle blockage ratio. As the longitudinal ventilation velocity in the interconnected tunnel increases, the convective heat transfer near the fire source increases, resulting in a decrease in the maximum temperature rise, and the effect of the blockage ratio on the maximum temperature rise is reduced. In this paper, a maximum temperature rise prediction model suitable for the case of blockage in the interconnected tunnel is proposed.

Experimental study on supercavitating flow by injection of 773 K ventilated gas

This study presents an experimental investigation of the characteristics of a ventilated supercavitating flow using high-temperature injection gas (VSHTG). Experiments are conducted in the cavitation tunnel located at the Chungnam National University in South Korea. Ventilation gas heated up to 773 K (500 °C) is injected behind a disk-type cavitator. The geometric characteristics of the VSHTG appearing with the temperature rise are captured using a high-speed camera, and the temperature characteristics inside the ventilated supercavity are measured using two K-type thermocouple temperature sensors. A theoretical physical model hypothesis is proposed, which is used to explain the VSHTG geometry variation with the increase in temperature and predict the thermodynamic effect caused by the injection high-temperature gas. Finally, the liquid–gas interface deformation of the VSHTG is examined using a colorimetric analysis. The results show that hot gas injection may increase the length of the supercavity and also make the liquid–gas interface unstable. A theoretical model of the thermodynamic effects is proposed and gives good agreement with the measurements. However, the findings should be further verified through additional studies including numerical analysis and experiments in a larger test section with the lower blockage ratio.

Effect of fluidic obstacles on flame acceleration and DDT process in a hydrogen-air mixture

Using solid obstacles to accelerate the deflagration to detonation transition (DDT) process induces additional thrust loss, and fluidic obstacles can alleviate this problem to a certain extent. A detailed simulation is conducted to investigate the effects of multiple groups of fluidic obstacles on the flame acceleration and DDT process under different initial velocities and gas types. The results show that, initially, the propagation of reflected shock wave formed by jet impingement is opposite to the flame acceleration direction, thus increasing the initial jet velocity will hinder the flame acceleration. Later, the vortex structure and enhanced turbulence can promote flame acceleration. As the flame accelerates, the virtual blockage ratio of the fluidic obstacles decreases, and increasing initial jet velocity or using reactive jet gases both affect the virtual blockage ratio. Further, increasing initial jet velocity or using reactive jet gases can shorten the detonation initiation time and distance. Compared with solid obstacles, it is concluded that fluidic obstacles can achieve faster detonation initiation with a smaller blockage ratio. Overall, the detonation phenomena in this study are all triggered by hot spots formed by the interaction between reflected waves and distorted flame, but the formation of reflected waves varies.

Hydrodynamic loads of the bridge decks in wave-current combined flows

This work uses a Large Eddy Simulation (LES) model and the Volume of Fluid (VOF) scheme to investigate the hydrodynamic loads on submerged rectangular decks in wave-current combined flows. The influences of current velocity, wave height, deck length, and blockage ratio on the wave loads are examined, and a modified Morison equation is used to predict the hydrodynamic forces on the deck. The simulation results demonstrate that the wave loads are linearly proportional to wave height H when H ≤ 0.4h, h is the water depth. By setting the reference velocity as Ur = (gH)1/2 for wave-induced flow, the hydrodynamic loads can be separated into a steady term (current-induced load) and an acceleration term (wave-induced load). The crucial hydrodynamic load comes from the surface pressures on the upper side of the decks, owing to the deck length being much larger than the deck thickness. In addition, the drag coefficient is independent of the wave height H, aspect ratio L/D, and the Keulegan-Carpenter number KC; while the lift coefficient depends on the submergence ratio S/D. Bridge engineers could easily use the modified Morison equation and the maximum drag coefficient CD = 2.73, lift coefficient CL = −2.05, the inertia coefficients CMx = 0.95 and CMz = 2.53 to compute the maximum loads against wave-current combined flows.

Multi-objective optimization of a tubular heat exchanger enhanced with delta winglet vortex generator and nanofluid using a hybrid CFD-SVR method

The present study improves the thermal-hydraulic performance of a circular tube equipped with a delta winglet vortex generator by providing a numerical model, investigating the effect of two additional parameters, and developing a support vector regression (SVR) for prediction and optimization. The additional parameters are the angle of attack and SiO2 concentration, while the data on the variation of thermohydraulic parameters with pitch and blockage is already available in the literature. The boundary layer disruption and swirling flow caused by the delta winglet result in heat transfer enhancement. The presence of such inserts also increases the pressure drop in the heat exchanger. The nanofluid further enhances heat transfer by boosting the thermophysical properties of the fluid. The hybrid method, CFD-SVR, is carried out for turbulent water flow at Reynolds numbers ranging from 4200 to 16200. The delta winglets are in pair arrangement with ten angles of attack from 25∘ to 70∘ with 5∘ increments. The addition of SiO2 with volumetric concentrations varies from φ=0 to 0.5% has been explored. The diameter of the tube (D) is 50 mm, while the pitch ratio (P/D) and blockage ratio (B/D) vary from 1 to 3 and 0.1 to 0.2, respectively. Finally, the data of this study and the previously published experimental data were fed into an SVR to find the best possible arrangement of delta winglet for a tubular heat exchanger. The highest achieved thermal enhancement factor (TEF) was found to be 2.18 with α = 25°, PR = 0.83, BR = 0.13, and φ = 0.5% at the least Re number.

Numerical investigation of the effect of reactive gas jets on the flame acceleration and DDT process

Jet obstacles can quickly induce the deflagration to detonation transition (DDT) process and reduce the thrust loss of an engine. However, there are few studies on the use of combustible premixed gases as the reactive jet obstacle. Based on the OpenFOAM platform, a detailed numerical investigation of the flame acceleration and DDT process is carried out with different initial jet velocities and the number of jet obstacles. The results show that, although the stronger flame generated by the reactive jet obstacles will reduce the virtual blockage ratio to a greater extent, the turbulence and combustion heat release effect generated by them still significantly promote the flame acceleration. In terms of flame acceleration, initially, increasing the initial jet velocity has no obvious effect on the flame acceleration. Later, turbulence and combustion heat release begin to dominate the flame acceleration and significantly promote the flame acceleration. Since the jets in this study adopt the same combustible premixed gases, increasing the number of reactive jet obstacles downstream does not improve the upstream flame acceleration. In DDT, increasing the initial velocity of the reactive jet can improve the detonation initiation, but the effect of the obstacle number on DDT is greatly affected by the initial jet velocity. Specifically, at a low initial jet velocity, more jet obstacles can significantly promote the detonation initiation, while at a high initial jet velocity, the enhanced turbulence caused by the jet is the dominant factor for the flame acceleration. Therefore, compared with the number of jet obstacles, the detonation initiation process of the premixed gases is more sensitive to the change in the initial jet velocity.